1. Field of Invention
This invention relates to the field of sensing aircraft and other objects and is part of the See and Avoid (SAA) function for manned aircraft and the Detect, Sense and Avoid (DSA) function for remotely piloted vehicles (RPVs) and unmanned aerial vehicles (UAVs). RPV is an older term for UAV. “UCAV” shall mean “Unmanned Combat Aerial Vehicle.” UCAV is also sometimes defined as an “Uninhabited Combat Aerial Vehicle.” UCAV is a UAV that is intended for use in combat. UAS means “Unmanned Aerial System.” UCAS means “Unmanned Combat Air System.” The characteristics all these vehicles have in common is that there is no human pilot onboard, and although they may be operated autonomously they can also be controlled by a remotely located operator or pilot. The term UAV shall be used as a generic term for such vehicles. Detect, Sense, and Avoid (DSA) is also commonly called Sense and Avoid (SAA) since “Detect” and “Sense” mostly mean the same thing. This invention is directed to the “See” in “See and Avoid” and the “Sense” in “Sense and Avoid.” It may also be used by ground stations to sense aircraft and other objects.
2. Prior Art
In an aircraft with the pilot onboard, Sense and Avoid is called See and Avoid. FAA Regulations do not give much guidance for seeing other aircraft.
Right-of-Way Rules: Except Water Operations 14 CFR §91.113(b) [IDS Cite 1]:
                (b) General. When weather conditions permit, regardless of whether an operation is conducted under instrument flight rules or visual flight rules, vigilance shall be maintained by each person operating an aircraft so as to see and avoid other aircraft. When a rule of this section gives another aircraft the right-of-way, the pilot shall give way to that aircraft and may not pass over, under, or ahead of it unless well clear.Right-of-Way Rules: Water Operations 14 CFR §91.115(a) [IDS Cite. 2]        (a) General. Each person operating an aircraft on the water shall, insofar as possible, keep clear of all vessels and avoid impeding their navigation, and shall give way to any vessel or other aircraft that is given the right-of-way by any rule of this section.When operating under Visual Flight Rules the idea is to look out small windows providing a limited field of view and hope you see any nearby aircraft in time to avoid a collision. This is made more difficult because of the wide range of aircraft sizes and speeds. (Is it a large aircraft far away or a small aircraft much closer?) This is even more difficult under instrument flight rules where there may be no visibility.        
Radar can be used to sense aircraft. Ground-based Radar allows Air Traffic Control (ATC) to direct aircraft in controlled airspace and keep aircraft safely apart. Military aircraft are generally equipped with onboard radar.
One type of collision avoidance system uses Secondary Surveillance Radar (SSR) where the Primary Surveillance Radar (PSR) used in air traffic control (ATC) detects and measures the position of aircraft and a secondary signal is transmitted that triggers a transponder in an aircraft that requests additional information from the aircraft itself such as its identity and altitude. Unlike Primary Surveillance Radar systems, which measure only the range and bearing to targets by detecting reflected radio signals, Secondary Surveillance Radar relies on its targets being equipped with a transponder which replies to each interrogation signal by transmitting its own response containing encoded data. U.S. Pat. No. 4,782,450 Method and apparatus for passive airborne collision avoidance and navigation issued Nov. 1, 1988 to Flax teaches that an aircraft can be equipped with a system that monitors the signals from the Secondary Surveillance Radar and the signals produced by each aircraft's transponders to produce its own onboard display of the locations of aircraft in the area. [IDS Cite 3]
The Traffic alert and Collision Avoidance System (TCAS) is an aircraft collision avoidance system designed to reduce the incidence of mid-air collisions between aircraft. It monitors the airspace around an aircraft by interrogating the transponders of other TCAS-equipped aircraft via the 1030 MHz frequency. It then uses the received transponder signals (via the 1090 MHz. frequency) to compute distance, bearing and altitude relative to its own aircraft. This interrogation-and-response cycle may occur several times per second. From the FAA's Introduction to TCAS II Version 7 [IDS Cite 4]                The TCAS Computer Unit, or TCAS Processor, performs airspace surveillance, intruder tracking, its own aircraft altitude tracking, threat detection, RA maneuver determination and selection, and generation of advisories. The TCAS Processor uses pressure altitude, radar altitude, and discrete aircraft status inputs from its own aircraft to control the collision avoidance logic parameters that determine the protection volume around the TCAS aircraft. If a tracked aircraft is a collision threat, the processor selects an avoidance maneuver that will provide adequate vertical miss distance from the intruder while minimizing the perturbations to the existing flight path. If the threat aircraft is also equipped with TCAS II, the avoidance maneuver will be coordinated with the threat aircraft.Where TCAS is relied upon to prevent mid-air collisions, an aircraft that does not have the equipment installed (or TCAS is broken or has been deliberately turned off) is a hazard to itself and other aircraft in the vicinity.        
The system by which an aircraft periodically transmits its identification, location, altitude, and heading is taught by U.S. Pat. No. 5,153,836 Universal dynamic navigation, surveillance, emergency location, and collision avoidance system and method issued Oct. 10, 1992 to Fraughton et al. [IDS Cite 5] and was materially adopted by the FAA as Automatic Dependent Surveillance-Broadcast (ADS-B). According to the article Gulf of Mexico Helo Ops Ready for ADS-B in Aviation Week & Space Technology (Feb. 26, 2007, page 56) [IDS Cite 6]:                By the end of 2010, FAA expects to have the ADS-B system tested and operationally acceptable for the NAS, with Houston Center providing services in the Gulf region. By 2013, all of the U.S. is scheduled to be covered with ground infrastructure.As with TCAS, where ADS-B is relied upon to prevent mid-air collisions, an aircraft that does not have the equipment installed (or ADS-B is broken or has been deliberately turned off) is a hazard to itself and other aircraft in the vicinity. ADS-B also comes with the risk that terrorists can use it to identify and track targets.        
A passive radar system is taught by U.S. Pat. No. 5,187,485 Passive ranging through global positioning system issued Feb. 16, 1993 to Tsui, et al. [IDS Cite 7] The patent teaches a method for determining the distance from a target to an observation station, using four GPS satellites as radiation sources, and a GPS receiver at the observation station to form a bistatic radar system, wherein an angle of arrival (AOA) of the target to the observation station has been measured first. Because the signal level from the GPS satellites is already low, the signal reflected from various objects is very low, requiring the use of a large antenna or more-powerful GPS satellites. See Test Results from a Novel Passive Bistatic GPS Radar Using a Phased Sensor Array by Alison Brown and Ben Mathews, NAVSYS Corporation. [IDS Cite 8]
There are other types of radar that attempt to keep the presence and location of the emitter from being detected. Examples are Spread Spectrum, Frequency Hopping, Ultra Wideband, and Noise Radar. Although there are differences between them, what they have in common is that they are designed to transmit a signal that cannot be detected except by the originating entity. As a result, target echoes also cannot be detected except by the originating entity. They generally do this by using a much wider bandwidth than a standard radar.
Spread Spectrum will be used here as an example. An example of Spread Spectrum Radar is taught by U.S. Pat. No. 5,724,041 Spread spectrum radar device using pseudorandom noise signal for detection of an object issued Mar. 3, 1998 to Inoue, et al. [IDS Cite 9].